1. Field of the Invention
The present invention relates to an active load circuit having improved operational stability, for example, when used as an active load circuit for a differential current amplifier.
2. Description of the Prior Art
In a prior art circuit including a differential amplifier consisting of a pair of transistors, which receive a differential input, and including a constant current source, the circuit also includes an active load circuit (or bias circuit) consisting of first (A and B) and second (C and D) pairs of transistors and a constant current source. The current output from the circuit is obtained from the current flowing from the constant current sources through the active load circuit transistors A, B, C, and D, and into and through the transistors in the differential amplifier. In the active load circuit, transistors A and B act as a current mirror circuit and an active load to the amplifier, while transistors C and D are emitter followers, i.e., their collectors are grounded, for the output stage. A disadvantage in the prior art circuit is that an offset current exists, and the circuit is balanced at a point shifted by this offset current. Another disadvantage is that a sufficient output current, i.e., a sufficient amplification factor, cannot be obtained, because of an insufficient change in the output current above the current differential.
In the above description, has been assumed that the amplification factors of transistors A, B, C and D are all equal. However, the amplification factors change, depending on the collector currents. The amplification factor has a convex curve characteristic, i.e., any increase corresponds to an increase in the factor in the collector current, that is, the curve reaches a predetermined value and then decreases. Thus, to make the amplification factors of transistors A, B, C and D all equal, the operation current for these transistors must also be made equal. The amplification factors of transistors A and B can be made equal by regulating their collector currents. This collector current regulation is effected by adjusting the constant current source. The amplification factor of transistor D can also be made equal to those of transistors A and B by regulating D's collector current through adjustments to the constant current source. However, the collector current of transistor C is small, because it is the sum of the base currents of transistors A and B, and it is difficult to adjust the amplification factor of transistor C as equal to those of transistors A, B and D, even if the characteristic of transistor C is equal to those of transistors A, B and D.
As can be seen from the above description, the offset current in this prior art circuit will be greater than required, and thus the circuit will probably become more unbalanced.